A large number of processes are known that operate by contacting gases with discreet catalyst particles. Processes of this type can be generally separated into those that use microparticles and those that use macroparticles. Macroparticle processes generally use extruded or formed particles having effective diameters greater than a 1/32 of an inch and are of a uniform or regular size. Processes that use macroparticles [hereinafter referred to as particulate material] routinely transfer the particulate material from one location to another. In many systems, such transport requires horizontal as well as vertical movement of the particulate material. Many of the macroparticles are relatively fragile and transport damages the particulate material by impingement or impact of the particles against each other or other surfaces. This attrition and breakage of regularly sized particulate material generates small fragments referred to as "fines" or fine particulate material.
Fines pose a multitude of operational problems for the processes that use regularly sized particulate material. For example, most processes that use particulate material of regular size contact the particles with a process fluid for the purpose of effecting a chemical reaction or imparting a physical change to the fluid or particulate material. In a number of these processes, it has been found necessary or desirable to transport the particulate material in and out of a contacting zone. In many such processes the contacting zone confines the particulate material in a retention bed or other space for holding the particulate material. The retention zone typically holds the particulate material at high density in a bed between two screens or other perforated device while a contacting fluid flows through the bed of particles. In such applications the minimum particle size must not exceed the size of the openings in the screen or other device to prevent particles from passing through openings with the fluid. Fines, when present, can pass through the opening of screens or other retaining elements and into the flow of process fluids from where they usually must be removed. In addition passage of the fines through the screen results in a loss of particulate material. Perhaps a bigger interference occurs when frees do not pass through, but plug portions of the screens or the bed thereby raising pressure drop. Where the particles are held in a dense bed, the sizing of the particles has an influence on the pressure drop. Particles of a uniform size will minimize pressure drop by permitting the maximum permeability of gas through the dense particle bed. Thus the presence of any frees raises overall pressure drop. Furthermore the presence of fines may also reduce the flowability of particulate materials and stop solids flow completely.
Many processes that use regularly sized particulate material are operated most efficiently by moving the particulate material while the process is in operation. Typical examples of such processes are catalytic processes for the conversion of hydrocarbons. In these processes, the catalysts are often composed of regularly sized particulate catalysts that become deactivated by the accumulation of coke deposits. The catalyst particles are regularly removed from the reaction zone to recondition the particles by removal of coke. Coke is normally removed from the catalyst by high temperature combustion, typically by contact with an oxygen-containing gas. Arrangements for continuously or semicontinuously removing such catalyst particles from a reaction zone to a regeneration zone for the removal of coke are well known.
Typical methods for transporting catalyst from one place to another in such an arrangement will use a system of conduits to move the particles from the bottom of the regeneration zone to the top of a reaction zone and from the bottom of a reaction zone to the top of a regeneration zone. This transport may occur in several stages wherein a number of retaining vessels hold catalyst particles as they are transported between reaction and regeneration portions of the process. The lifting of catalyst particles from one process region to another is usually done with some form of pneumatic conveying wherein a gas stream having sufficient velocity to lift the catalyst particles transports it upwardly for transport and disengagement into another series of vessels. Such systems of transport are well known and commonly use a number of pipe elbows and valve arrangements to direct and control the movement of the catalyst particles. As mentioned previously breakage of catalyst particles and the generation of fine particulate material has been a regularly observed and unwanted consequence of transporting catalyst particles in such systems.
The fines generated by such transport not only interfere with the operation of the beds but can also raise pressure drop for gases passing through the piping elements. High pressure drops associated with the transfer of particulate material adds to operational costs of the process and can interfere with maintenance of desirable operational conditions. As a result, methods have been sought to reduce the required pressure drop for conveyance of macroparticles in systems that contact gas and particulate material.
Long radius elbows have typically been used as standard piping elements in the conveyance of particulate matter. The long radius elbows have been employed to minimize attrition and pressure drop for the pneumatic conveyance of the particulate material.
It has been found that the pressure drop of long radius elbows is the result of disengagement of the catalyst particles from the gas flow in the long radius elbow. This disengagement of the gas flow causes slumping of the catalyst within the elbow and reacceleration of the catalyst as it continues transport through the elbow. This slumping and reacceleration apart from increasing pressure drop also creates a churning motion in the elbow that leads to high catalyst attrition.